Purification of alkali metal hydroxides



Patented Oct. 11, 1938 UNITED STATES PURIFICATION OF ALKALI METALHYDROXIDES Henry Mills Spittle, Northwich, England, assignor to ImperialChemical Industries Limited, a corporation of Great Britain No Drawing.Application May 20, 1937, Serial N0. 143,854. In Great Britain May 26,1936 10 Claims.

This invention relates to the purification of solutions of alkalimetalhydroxides.

Alkali metal hydroxide solutions obtained by certain industrialprocesses, such as the customary lime causticization process, frequentlycontain appreciable quantities of undesirable silica, alumina andmanganese compounds.

An object achieved in the present invention is the .provision of aconvenient and effective 10 method of removing impurities fromcaustic'alkali solutions containing silica. Alumina and manganesecompounds are also removed. The invention is based on my discovery thatsilica, lime and the reactive forms of Fezos in caustic alkali solutionproduce a calcium feric silicate insoluble in the alkali solution. As anempirical matter the best results are obtained Where the'reactive bodiesare present at the time of action in the proportion of at least 1.3molecules of a reactive form 20 of F8203 per molecule of SiOz and aboutmoleculesof CaO. 'Idealized, the precipitate should have the, formula1SiOz.1.3Fe2Os.5CaO. Actually, howevenalumina and manganese present inthe alkali solution join the precipitate and modify 25 the compositionof the precipitate somewhat. There is practically always some alumina inthe precipitate and very often some manganese. The methodisparticularly' applicable to the dilute caustic alkali solutionsobtained in causticizing 30 carbonate solutions with lime. It isparticularly convenient with caustic soda solutions. Most of thesesolutions contain a littleiron together with much silica. It is usuallynecessary to add more iron and more lime.

. In practical embodiments of the present invention alkali metalhydroxide solutions containing up to about 45-50 per cent by weight ofthe alkali metal hydroxide are digested with, suitable quantities of twoor more reagents, at least one re- 40 agent being selected from each ofthe following groups:(l) the hydrated oxides, hydroxides and salts. oftrivalent iron, including salts of acids derived from ferric oxide, e.g. sodium ferrite; (2) the oxides, hydroxides and salts of cal- 45 ciumand other alkaline earth metals. The silicon compounds; as a result ofthis, treatment, come down in a precipitate which can be removed byfiltration, centrifuging -or settling. Aluminium and manganese'compoundspresent in the initial 50 solution are also precipitated as aresult of the treatment, although the degree of removal of thesecompounds may not be so high as in the case of silicon compounds. Theprecipitate formed by the joint use of a 'calcium'and an'iron I compoundis probably. a complex calcium ferric silicate containing varyingquantities of alumina (usually in the form of a mixed crystal) accordingto the nature of the initial solution.

For the purpose of the invention most reactive trivalent iron compoundsappear to be effective. 6 Naturally one should not use iron compoundsthat are extremely insoluble and non-reactive,

e. g. unhydrated ferric oxide, or iron compounds in which the iron iscontained in avery stable complex ion, as such compounds cannot be ex-10 pected to take part in the reaction or reactions leading to theprecipitation of the silicon com{ pounds. It will also be understoodthat com pounds containing silicon may not be suitable as their usemight introduce more silicon than would be removed in the precipitate. i

As the trivalent iron compound I can employ a hydrated ferric oxide,ferric hydroxide, ferric sulphate, ferric chloride or sodium ferrite. Asthe alkaline earth metal compound I can employ a compound of calcium, e.g. the oxide, bydroxide, carbonate or sulphate, not merely because ofthe relative cheapness and availability of such calcium compounds, butbecause of their very much greater purification efficiency when comparedwith barium strontium or magnesium compounds. I have obtained goodresults when using ferric hydroxide or ferric sulphate in conjunctionWith calcium hydroxide or carbonate. The reagents may-be added in eitherorder or simultaneously. I usually carry out the digestion i at anelevated temperature, e. g. 100 0., asI find that the reaction isthereby accelerated.

The trivalent iron compound employed may be added as such or formed insitu, if desired by oxidation of a divalent iron compound. Thus I mayuse ferric hydroxide or hydrated ferric oxide either as such or formedby reaction of a ferrous or ferric salt with a solution or suspension ofa hydroxide or a carbonata'with in-.- 40 jectionof air in. the case of aferrous salt. Alternatively, the ferric compound for use in the processcan be recovered from the precipitate formed when carrying out theprocess of the invention; I

The amounts of reagents required depend on i the form in'which they areadded and uponthe amounts of silica, alumina or manganese oxide to beremoved. Under suitable conditions and ordi- 50 nary type of liquors percent or more of small amounts of silica can be removed by using at least1.3 moles of iron-compound, reckoned as R203, per mole of silica, and atleast 5 moles of calcium compound, reckoned as CaO, per mole of 56silica, but the invention is in no way limited to these proportions.

The alkali metal hydroxide solution may be relatively dilute, e. g. itmay contain about per cent by weight of alkali metal hydroxide. It maycontain certain substances such as carbonate, sulphate and'chloride,which are often present in caustic liquors, without preventing theprecipitation of the silicon compound. The solution may also be in theform of a magma or slurry containing solid calcium carbonate orhydroxide (e. g. a magma obtained in a lime causticizing process) andthe said calcium carbonate or hydroxide may be utilized as the secondreagent, only a reagent or reagents of the first group being added tothe solution.

The following examples illustrate, but do not limit, the invention.

Example 1 The initial solution contains 11 per cent by weight of NaOI-Iper 100 parts of solution and a silica content, calculated as S102, of400 parts per million. To 100 parts by weight of this solution, at atemperature of 100 C., 0.5 part of Ca(OI-I)2 is added with stirring,then 0.34 part of ferric sulphate as a 20 per cent solution. Thesolution is maintained at 100 C. for about 2 hours, with stirring, afterwhich the precipitate is removed from the liquor in any suitable manner.The solution now contains only about 40 parts per million of silica, i.e. 90 per cent of the silica has been removed. In addition, about 50 percent of the alumina content of the solution, initially about 300 partsper million, is removed.

Comparative experiments showed that neither Ca(OH)2 nor ferric sulphatealone, in the above quantities, would reduce the SiOz content to muchbelow 300 parts per million. Further, even when using alone such a largequantity of ferric sulphate as 3.4 parts by weight very much less silicawas removed than when using both calcium hydroxide and ferric sulphatein the above mentioned quantities.

Example 2 The initial solution contains about 11 per cent by weight ofcaustic soda, silicon (calculated as SiOz) 2'75 parts per million andmanganese 0.4 part per million. The liquor is treated with hydrated limeand ferric sulfate (used as a 20 per cent solution) in the proportion of1.5 grams Ca(OH)2 and 1 gram ferric sulfate for 150 cc. of liquor. Theresulting precipitate is removed. The purified solution has a silicacontent of 15 parts per million (calculated as SiOz) and a manganesecontent of0.19 part per million.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it'is to beunderstood that I do not limit myself to the specific embodimentsthereof except as defined in the appended claims.

I claim:

1. In the purification of caustic alkali sol tions containing silica inamount in excess of,

the amount which will combine with the ferric iron present, the processwhich comprises adding to said solution a trivalent iron compound inamount sufficient to provide, in connection with any trivalent ironalready present, a molecular ratio of ferric iron calculated as FezOs tosilica of at least 13:1 and digesting the mixture thus formed, at anelevated temperature in contact with a material selectedfrom the groupconsisting of oxides, hydroxides and salts of the alkaline earth metals,thereby forming an insoluble complex ferrisilicate and removing theferrisilicate thus formed.

2. In the purification of caustic soda liquors containing silica inamount in excess of the amount which will combine with the ferric ironpresent, the process which comprises adding to said solution a trivalentiron compound in amount sufficient to provide in connection with anytrivalent iron already present, a molecular ratio of ferric ironcalculated as F6203 to silica of at least 1.3:1 and digesting themixture thus formed, at an elevated temperature in contact with amaterial selected from the group consisting of oxides, hydroxides andsalts of the alkaline earth metals, thereby forming an insoluble complexferrisilicate, and removing the ferrisilicate thus formed.

3. In the purification of caustic alkali liquors containing up to 50 percent by Weight of alkali hydroxide and containing silica'in an amount inexcess of the amount which will combine with the ferric iron present,the process which comprises adding to said solution a trivalent ironcompound in amount sufiicient to provide, in. connection with anytrivalent iron already present, a molecular ratio of ferric ironcalculated as F8203 to silica of at least 13:1 and maintaining themixture thus formed, at an elevated temperature in contact with amaterial selected from the group consisting of oxides, hydroxides andsalts of the. alkaline earth metals, thereby forming an insolublecomplex ferrisilicate, and removing the ferrisilicate thus formed.

4. In the purification of caustic soda liquors containing silica inamount in excess of the amount which will combine with the ferric ironpresent, the process which comprises adding to said solution'a trivalentiron compound in amount suflicient to provide, in connection with anytrivalent iron already present, a molecular ratio of ferric ironcalculated as F9203 to silica of at least 1311 and maintaining themixture thus formed, at an elevated temperature in contact with amaterial selected from the group consisting of the oxide, hydroxide andsalts of calcium, thereby forming an insoluble complex ferrisilicate,and removing the ferrisilicate thus formed.

5.,In the purification of caustic soda liquor containing silica inamount in excess of the amount which will combine with the ferric ironpresent, the process which comprises adding to said solution a divalentiron compound and oxidizing the said compound sufficiently to provide inconnection with any trivalent iron already present, a molecular ratio offerric iron calculated as F6203 to silica of at least 1.3: 1 anddigesting the mixture thus formed, at an elevated temperature in,contact with a material selected from the group consisting of oxide,hydroxide and salts of calcium, thereby forming an insoluble complexferrisilicate, and removing the ferrisilicate thus formed.

6. In the purification of caustic soda liquor containing silica inamount in excess of the amount which will combine with the ferric ironpresent, the process which comprises adding to said solution a trivalentiron compound in amount sufficient to provide in connection with anytrivalent iron already present, a molecular ratio of ferric ironcalculated as F6203 to silica of at least 1.321 and maintaining themixture thus formed, at an elevated temperature in contact with amixture selected from the group consisting of the oxide, hydroxide andsalts of calcium in amount sufficient to provide at least mols ofcalcium calculated as CaO per mol of silica, thereby forming aninsoluble complex ferrisilicate and removing the ferrisilicate thusformed.

'7. In the purification of caustic alkali liquors resulting from thecausticization of alkali carbonates with lime by removing silica andother impurities therefrom, the process which comprises adding to such aliquor a reactive form of ferric iron and a reactive calcium compound,the amount of each being sufiicient to bring down silica, ferric ironand lime in the solution as a precipitate having the approximatecomposition SiO2.1.3FezO3.5Ca0 and removing the precipitate from thepurified liquor.

8. In the purification of caustic alkali liquors containing silica inamount in excess of the amount which will combine with any ferric ironalready present in the solution, the process which comprisesincorporating in such a liquor a reactive calcium compound selected fromthe group consisting of the oxide, hydroxide and salts of calcium, andan iron compound of such character as Will yieldtrivalent iron in theliquor by dissolution, the amount of iron and calcium being sufiicientto bring down silica, ferric iron and lime as an insoluble ferrisilicateprecipitate containing CaO, S102 and F8203, with the Fezoszsioz ratio atleast 13:1; and removing the precipitate from the purified liquor.

9. In the purification of caustic alkali solutions containing silica,the process which comprises subjecting such solutions, in contact withsubstantial amounts of a material selected from the group consisting ofalkaline earth metal oxides, hydroxides and salts, to the action of anadded ferric compound, said ferric compound being present in sufiicientamount to provide a molecular ratio of ferric iron, calculated as FezOs,to said silica greatly in excess of 1:1, and removing the precipitateformed.

10. In the purification of caustic alkali solutions containing silica,the process which comprises subjecting such solutions, in contact with asubstantial amount of a calcium compound selected from the groupconsisting of calcium oxide, hydroxide and carbonate, to the action ofan added ferric compound, said ferric compound being present insufiicient amount to provide a molecular ratio of ferric iron,calculated as FezOs, to said silica greatly in excess of 1:1, andremoving the precipitate formed.

- HENRY MILLS SPITILE.

